Depth cameras often illuminate a scene with modulated laser light. Measured depth precision improves with increased modulation frequency, increased light power, and increased modulation depth. In many cases, scene illumination consumes a substantial portion of an overall depth camera power budget. Accordingly, it can be appreciated that an efficient, high frequency, high power laser diode driver circuit is a key component in a depth camera.
High power solid state laser diodes emit light power proportional to their junction current. For a high power edge emitting infrared laser diode, a voltage of close to 2V is required. For a laser diode including high powered vertical-cavity surface-emitting lasers (VCSELs) the forward voltage can be as high as 2.8V.
Exemplary prior art current mode laser diode drivers are shown in FIGS. 10 and 11, with FIG. 10 illustrating a single-ended configuration laser diode driver 1004, and FIG. 11 illustrating a differential configuration laser diode driver circuit 1104. The laser diode drivers of FIGS. 10 and 11 both control laser diode power by adjusting a current source Ictl. In FIG. 10, a single-ended voltage modulation signal Vmod modulates the laser diode LD by controlling a switching transistor Msw to switch the current flowing from the laser diode to the current source Ictl. In FIG. 11, differential voltage modulation signals, Vmod and Vmod_bar, modulate the laser diode LD by controlling differential switching transistors Msw1 and Msw2 so that current flow in the current source Ictl is switched at the laser diode LD but is continuous at the current source Ictl or single-ended so that the current source current Ictl is switched along with the current to the laser diode LD. The single-ended approach of FIG. 10 is more power efficient, but is more difficult to implement at high switching frequencies than the differential approach of FIG. 11.
An advantage of the prior art current mode laser diode drivers 1004 and 1104 of FIGS. 10 and 11 is that laser diode power is easily controlled by adjusting the current in the current source Ictl. Also, high switching speed is readily obtainable in the differential configuration of FIG. 11.
A disadvantage of the prior art current mode laser diode drivers 1004 and 1104 of FIGS. 10 and 11 is that they have poor power efficiency because only a small part of the power supply voltage drop is across the laser diode LD. For instance, if a power supply of 5V is used and a laser voltage of 2V is assumed, the resulting efficiency is only about 40 percent. Power efficiency is further reduced by voltage drop across the transistor switches, Msw in FIG. 10, and Msw1 and Msw2 in FIG. 11. Another significant shortcoming of the prior art current mode laser diode drivers 1004 and 1104 is that it is difficult to create an accurate current reference and current mirroring setup. The resulting current is often modulated by the power drawn by the laser diode LD and these current fluctuations can cause positive feedbacks resulting in severe peaking, which is undesirable.
Laser diode drivers, such as but not limited to those of FIGS. 10 and 11, encounter the problem of inductive kickback. When the current in the laser diode is switched off, inductance in the laser diode package and the driver package fights the turnoff and boosts the cathode voltage Vk of the laser diode LD. Left uncorrected, this voltage kickback will boost the cathode voltage Vk well above the power supply voltage Vdd, which can potentially damage circuitry of the laser diode driver.
A typical solution to this problem is to add an external clamping diode, Dclamp, as shown in FIGS. 10 and 11. If this clamp diode is sufficiently fast and has sufficient current carrying capacity, the voltage at Vk will only rise a little above the power supply voltage VL and the driver chip will be spared most of the damage. However, in practice, it is difficult to find diodes with sufficient speed and current capacity for high-speed and high-power applications, such as these. Moreover, the clamp diode and board parasitic inductances should be minimized for this solution to be effective. However, since these clamping diodes normally are on a printed circuit board they have to contend with the package and board inductance, which renders them to a great extent less effective.
Another problem encountered in laser diode driver circuits for depth cameras is variation in the latency through the driver circuits over process, temperature, and voltage. Uncompensated latency changes will result in significant errors in measured depths. For example, in one configuration, a change in latency of 6 ps can produce a measurement error of ˜1 mm. A high-power driver in a standard configuration will require a large number of buffers to boost the drive current to the desired level. Such buffers will have insertion delay and this delay will vary by much more than 6 ps over process, temperature, and voltage.
The main disadvantages of the prior art current mode laser diode driver circuits of FIGS. 10 and 11 are the large required die area, poor high frequency operation in the single-end configuration of FIG. 10, and poor power efficiency.